1. Field of the Invention
The invention relates to glass-ceramic plates designed to cover heating elements. More precisely, the invention is directed toward glass-ceramic plates which constitute part of cooking tops. Such cooking tops are provided in particular with heating elements such as heat sources of radiant or halogen-lamp type and with control and regulation means allowing the power thereof to be varied.
2. Discussion of the Background
For such control and regulation means there are commonly employed knobs fixed on spindles passing through holes in the glass-ceramic plate. This solution is not entirely satisfactory. In the first place, the provision of holes in the plate, under conditions which preserve sufficient mechanical strength thereof while limiting manufacturing defects and therefore the proportion of rejected products, necessitates the use of complex processes. In the second place, such an embodiment of the control means does not permit complete cleaning of the plate, because regardless of the leaktightness of the interface between the knob and plate, the peripheral rim of the knob necessarily forms a sharp edge or defines a slit, which inevitably will attract and possibly trap dust, small crumbs, etc.
One solution consists in replacing the classical control knobs by sensing keys in accordance with the teaching of European Patent Application EP 0443924 A1. The control and regulation means for a cooking or warming means comprises, especially as described in that document, a pair of keys, one of which is used to increase the power delivered to the associated heat source each time the user's finger makes contact with this key, while the other is used to reduce the power of this same heat source in steps. According to an alternative, there can be provided a plurality of keys corresponding to distinct heating powers. In order to define precisely the positioning of the keys and their allocations to the different heat sources, the document suggests forming marks on the top surface of the glass plate by a silk-screen or other process. It thus involves a basically visual mark; in fact, although screen-printed patterns stand out very slightly (for example, by 4 .mu.m at most) from the plane of the plate and constitute a differentiated surface condition thereof, they do not permit uniquely tactile marking of the control means and of the manner in which they can be used, even by an experienced person, blind or otherwise.
In addition, screen-printed marks are not desirable with regard to cleaning the plate. They actually have a tendency to retain dust and to be progressively destroyed under the abrasive action of the cloth or sponge and by chemical attack due to the cleaning product.
As a remedy, European Patent Application EP 0464323 A2 proposes, for marking the contours of the heating zones on a glass-ceramic plate, replacing the screen-printed patterns by bands of parallel recessed grooves on the surface of the plate. Such a mark has the advantage of being almost indelible, and is designed in particular to be unaffected by repeated sliding movements of various utensils (pans, casserole dishes, etc.) thereabove. Such marks permit visual marking. However, tactile marking, for example for a blind user, is not described in EP 0464323, presumably because it would be unsafe in view of the close proximity of the cooking or warming means and the risk inherent therein.